Hyperlipidemia or increased blood lipid level is a major cause for cardiovascular diseases and arteriosclerosis. Representative examples of hyperlipidemia are hypercholesterolemia, familial dysbetalipoproteinemia, diabetic dyslipidemia, nephrotic dyslipidemia and familial combined hyperlipidemia.
Several types of agents for lowering the blood lipid level have been developed to treat hyperlipidemia or hypercholesterolemia. Typically, these agents reduce the synthesis of lipoproteins or lipids in serum, or facilitate the removal of lipoproteins or lipids from serum or plasma. Among these agents, inhibitors of HMG-CoA reductase, a rate-limiting enzyme in the biosynthetic pathway of cholesterol, have been developed to lower the concentration of lipoproteins or lipids in serum. Exemplary HMG-CoA reductase inhibitors are: mevastatin (U.S. Pat. No.: 3,983,140), lovastatin also called mevinolin (U.S. Pat. No.: 4,231,938), pravastatin (U.S. Pat. Nos.: 4,346,227 and 4,410,629), lactone of pravastatin (U.S. Pat. No.: 4,448,979), velostatin and simvastatin, also referred to as synvinolin (U.S. Pat. Nos.: 4,448,784 and 4,450,171), rivastatin, fluvastatin, atorvastatin and cerivastatin.
The HMG-CoA reductase inhibitors have been widely used for treating hyperlipidemia for several decades to lower total in vivo concentration of cholesterol and LDL-cholesterol (Grundi, S. M, et al., N. Engl. J. Med. 319(1): 24-32, 25-26 and 31, 1998). The synthesis of mevalonate by the action of HMG-CoA is an early step in the biosynthetic pathway of cholesterol, and the HMG-CoA reductase inhibitor lowers total concentration of cholesterol and LDL-cholesterol in serum by inhibiting the synthesis of mevalonate (Grundi, S. M. et al., N. Engl. J. Med. 319(1): 24-32, 25-26 and 31, 1998).
However, most of such HMG-CoA reductase inhibitors are administered in the form of rapid release formulations, which causes side effects such as hepatoxicity, muscular disorder and rabdomyolysis (Garnet, W. R. et al., Am. J. Cardiol. 78: 20-25, 1996; The lovastatin pravastatin study group, Am. J. Cardiol. 71: 810-815, 1993; Duzovne, C. A. et al., Am. J. Med. 91: 25S-30S, 1991; and Mantel, G. M. et al., Am. J. Cardiol. 66: 11B-15B, 1990).
Accordingly, there has been a need to develop a sustained release formulation of the HMG-CoA reductase inhibitor to prevent or alleviate the side effects induced by the rapid release of HMG-CoA reductase inhibitor. Many studies on the sustained release formulation of HMG-CoA reductase inhibitor have suggested that most of the HMG-CoA reductase inhibitors absorbed in the body are metabolized in the liver (85% and more) while only 5% or less account for those transferred to the systemic circulation system. Thus, a bioavailability of the HMG-CoA reductase inhibitor to the systemic circulation system is poor. Also, as the HMG-CoA reductase inhibitor exerts its enzymatic activity mainly in the liver, it is important to understand the pharmacokinetics in the liver as well as its bioavailability. The rapid release formulation of HMG-CoA reductase inhibitor shows the dose-dependent nonlinear pharmacokinetics, but cannot maintain its therapeutic effect for a long time because of the prolonged clearance half-life caused by saturation (capacity-limited) phenomenon present during a hepatic metabolism. However, when administering the sustained release formulation of HMG-CoA reductase inhibitor, although the blood concentration of HMG-CoA reductase inhibitor may be lower than that of the rapid release formulation due to the hepatic metabolism, there is no occurrence of the saturation due to its low blood concentration. According to the latest studies, it has been reported that the sustained release formulation of HMG-CoA reductase inhibitor shows equal or slightly lower bioavailbility than the rapid release formulation in both acidic and lacton types. However, its drug delivering efficiency to a target site is shown to be superior to that of the rapid release formulation (John R, Amer. J. Cardio. 89: 15, 2002). Accordingly, the sustained release formulation is capable of more effectively lowering the LDL-cholesterol level in blood than the rapid release formulation (Monique P, Am. J. Drug Deliv. 1(4): 287-290, 2003).
The present inventors have therefore endeavored to solve the problems of the rapid release formulation of HMG-CoA reducatse inhibitor previously reported and developed a new sustained release formulation of HMG-CoA reductase inhibitor having an improved bioavailability with minimal side effects by keeping the amount of HMG-CoA reductase inhibitor at a constant level in blood through a slow and uniform release mechanism thereof.